Previously, NULL files (*7.6) could only be read as data using index variables (AR function). The routine:
NULVAL(F,FN,XN,XVAL)
will set XVAL to the value of the argument of the XN-th NULL in the NULL file called F, number FN.
Hence:
NULVAL(ANAME ('FRED'),1.0,3.0,XVAL)
will set XVAL to 6.0, taking the file defined in section 1.1.
The routine NULSET (*7.7) allows NULL files to be obeyed by giving meaning to the NULL. The argument of NULSET has been changed from a display routine name to a display routine number, as defined in (*A3).
Thus:
NULSET(32.0)
will set NULLs to be equivalent to TOXY and:
NULSET(33.0)
will set NULLs to be equivalent to VEC.